Myelin

Myelin is the fatty material that wraps around the long projections (axons) of many neurons. It is produced by specialised glial cells: oligodendrocytes in the brain and spinal cord, and Schwann cells in the peripheral nervous system.

It is what makes white matter white. Without myelin, the brain's wiring would still work, but slowly and inefficiently.

What it does

Myelin functions as electrical insulation. It does not cover the axon continuously. Instead, it forms segments separated by tiny gaps called nodes of Ranvier. Signals jump from node to node, a process called saltatory conduction. This:

• Speeds transmission roughly 50 to 100 times compared with an unmyelinated axon of similar diameter
• Reduces the energy cost of signalling
• Improves the precise timing of signals between regions

A signal that takes 200 milliseconds to travel along an unmyelinated nerve can travel the same distance in 2 milliseconds along a myelinated one.

When it fails

Myelin damage is the central feature of several conditions:

• Multiple sclerosis: the immune system attacks oligodendrocytes, producing patchy demyelination throughout the central nervous system
• Guillain-Barré syndrome: the same process in the peripheral nerves
• Leukodystrophies: rare genetic disorders of myelin metabolism
• Some traumatic brain injuries: shearing forces can damage myelinated tracts (diffuse axonal injury)

White matter hyperintensities seen on FLAIR MRI often reflect chronic demyelination, gliosis, or small-vessel damage to myelinated tissue.

Myelin and learning

Myelin was once thought to be relatively static after development. Newer evidence shows it can be modified by experience. Learning a new motor skill, for example, can increase myelination of relevant tracts. This is one of the mechanisms behind neuroplasticity in adults.